This invention relates to the removal of pollutants from the exhaust gases resulting from the incineration of wastes. The invention particularly relates to the removal of acid gases, toxic organic and toxic metal compounds from biomedical waste incinerator exhaust gases by a method and apparatus that minimizes emission of contaminants to the environment. This invention further relates to the removal of pollutants from waste incinerator exhaust gases by means combining both dry and wet scrubbing, but which avoids the production of a contaminated liquid byproduct waste stream and its release to the environment.
The incineration of medical and municipal wastes, which typically contain halogenated compounds in the form of plastics and solvents, yields relatively large amounts of TOC in the off-gas, as well as the acid gases, HCl and SO.sub.2, and toxic metals. In the incineration of biomedical wastes, negligible amounts of SO.sub.2 are typically evolved, and the major acid gas in the exhaust gases is HCl. Three major types of prior art methods have been employed for the removal of these pollutants from incinerator flue gases: spray-dryer/solids-removal, dry alkaline solids gas contacting/solids-separation and wet scrubbing. Each of these methods are deficient in effecting the removal from the gas of one or more classes of the pollutants in incinerator off-gases.
Wet scrubbing comprises contacting of the combustion gases with an aqueous scrubbing solution in an efficient gas-liquid contactor. Typically, wet scrubbers use an alkaline solution of alkali metal or alkaline earth carbonate or hydroxide for neutralization of the absorbed acid gases. Wet scrubbers are highly efficient devices for the removal of the acid gases and the TOC components in the gas, but do not provide satisfactory removal of the particulates or toxic metals. Wet scrubbers also suffer from the fact that they product a contaminated liquid blowdown stream.
Spray drier/absorption methods for incinerator off-gas treatment are of fairly recent origin, and a review of the various forms of this technology has been given by Kroll and Williamson, "Application of Dry Flue Gas Scrubbing to Hazardous Waste Incineration", Journal of the Air Pollution Control Association, pp. 1258-1263, Vol. 36, No. 11, November, 1986. True dry scrubbing, i.e., removal of pollutants by means of dry aklaline powder contacting, is described by Muzio and Offen, in the Journal of the Air Pollution Control Association, Vol. 37, pp. 642-653, May, 1987. Dry scrubbing comprises the introduction of a finely-divided alkaline solid material into intimate contact with the flue gas, followed by solids separation from the gas. The preferred reactant is hydrated lime (calcium hydroxide) although where economically or chemically justified, sodium carbonate or bicarbonate may be used as the alkaline reactant.
In the present context, "dry scrubbing" shall mean means for contacting hot combustion gases with a finely-divided alkaline solid at temperatures above the dewpoint of the gas, followed by a solids collector such as a fabric filter (baghouse) or electrostatic precipitator (ESP). Also in the present context, "spray drier scrubbing" shall mean means for contacting hot combustion gases with an aqueous alkaline solution or dispersion in a spray dryer, follower by a fabric filter or ESP collector.
Dry and spray-drier scrubbing processes, with ESP or baghouse filtration as solid separation means, can remove almost all of the particulates, as well as significant fraction of the acid gases and toxic metals in the flue gas from biomedical waste incinerators. For example, conventional dry scrubbing with hydrated lime can remove major fractions of the HCl in the gas, and dry scrubbing with sodium bicarbonate can remove major fractions of the SO.sub.2, as well as the particulate fraction. However, these methods fail to effectively remove the TOC class of pollutants and volatile toxic metals such as mercury.
The residual TOC levels remaining in the gas in both dry and spray drier scrubbing are typically not acceptable for environmental discharge. In the case of spray drier scrubbing, the addition of activated carbon in, or after, the spray drier scrubbing stage to effect more efficient dioxin and mercury removal is taught by the prior art. Moller, et al., in U.S. Pat. No. 4,889,698, reviews the art of activated carbon addition in, or downstream of, a spray drier scrubber for mercury and dioxin removal, and teaches the use of a powdered activated carbon as a supplement to spray drier technology to improve the removal of dioxins and mercury. The efficient removal of the pollutants with powdered activated carbon adsorbent in spray drier scrubbing occurs when sufficient water is evaporated to cool the flue gas to 100.degree. C.-130.degree. C. The latter gas-moisturizing condition engenders corrosion and/or failure of the solids separation equipment. Further, the use of activated carbon for dioxin/mercury removal from the gas phase is uneconomic, and requires secondary treatment of the contaminated carbon prior to disposal.
The deficiencies of the spray drier scrubber or dry powder scrubber can be compensated for by adding a wet scrubber gas treatment stage downstream of the dry scrubber stage. The wet scrubber will effectively remove the residual acid gases and TOC's that typically penetrate the dry treatment stage. Thus, an emission control system comprising a dry scrubber, or a spray drier/absorber, and a wet scrubber operating downstream on the filtered gas, will provide high removal efficiency for all classes of the toxic pollutants; acid gases, toxic metals, particulates and TOC.
The wet scrubber stage of an incinerator emission control system is typically operated with a scrubber recycle liquor solution containing an alkaline material, such as caustic, to neutralize the absorbed residual acid gases. The use of caustic neutralization in a wet scrubber has several disadvantages. Combustion gases from an incinerator contain carbon dioxide as a principal product of combustion, and, as an acid gas, carbon dioxide is absorbed by the alkaline scrubber solution. Unless the scrubber makeup water supply is "soft" water, that is, pretreated to remove calcium hardness, then insoluble calcium carbonate precipitates from the scrubber solution. This precipitate tends to foul and plug the scrubber, and continuous stable operation demands the use of softened water, which can become very expensive. Additionally, alkaline neutralization of the scrubbing liquor generates a solution of sodium chloride from the HCl content of the liquor. In order to keep the liquid solution from salting up, a liquid blowdown stream is typically taken off the wet scrubbing liquor recycle at a rate sufficient to avoid the precipitation of salt in the recycle solution. In most cases, the byproduct liquid stream from this wet scrubbing stage which is heavily contaminated with TOC's is passed either to a municipal or in-plant waste treatment system, which does not remove or destroy the major liquid contaminants.
The hazardous and toxic pollutants removed from the gas in the wet scrubber are thus eventually discharged to the environment in a form which can readily enter drinking water or recreational water supplies and thus expose the general public to these hazardous pollutants. In terms of dioxin content, the Environmental Protection Agency has recommended a 0.014 parts per quadrillion limit for the content of receptor bodies of water, so that waste streams discharging into existing water bodies must necessarily be very low in dioxin content. The contaminated liquid blowdown discharge is one of the major drawbacks to the use of wet scrubbers in waste incinerators off-gas cleanup.
Kokkonen, et al, in U.S. Pat. No. 5,002,743, teach the evaporation of alkaline wet scrubber liquor from the flue-gas scrubbing stage. Evaporation of the wet scrubber blowdown does not destroy the TOC contained in the liquor, it merely concentrates these toxic compounds, as well as the contained salt and the concentrated waste must be treated as a highly hazardous material. Evaporation within the air pollution control train is self-defeating, inasmuch as such a practice would quickly raise the gas concentrations of TOC, saturate the wet scrubber liquid with TOC, and discharge high TOC levels in the gas leaving the scrubber. Thermal destruction is known to be the only practical option for TOC-containing streams. However, incineration of the blowdown alkaline wet scrubber solution to thermally destroy the TOC content is not feasible because the salts and liquid alkalinity would rapidly attack and destroy the refractory lining of an incinerator. Continued discharge of the waste liquid stream from such wet scrubbing stages therefore represents an ongoing ecological threat which has not been satisfactorily resolved.